The increasing dynamic requirements, such as monotonic start-up, recovery after short-circuit, load transient performance, have led to the re-partitioning of the control circuits of many modern isolated switched mode power supplies from the primary side to the secondary side.
One problem with this control circuit, now placed on the secondary side, is that it has to be powered from the primary side and the input voltage has to be monitored accurately, since it is used in the control of the main converter.
The control circuit is often powered via a small housekeeping or auxiliary converter There are numerous of ways of designing such a housekeeping or auxiliary supply for an isolated switched mode power supply.
Fly-buck converters have been proposed, see US 2011/0090724 A1, but the high voltage isolation requirements cause problems when designing the fly-buck converter; it may be challenging to supply the primary side with regulated power.
An alternative is to use an auxiliary winding in, or tap off power from, the main converter of the isolated switched mode power supply. This requires, however, that the main switches of the main converter are switching, which causes problems during pre-bias starts and fault handling, when the output of the main converter has to be turned off, but the housekeeping power on the secondary side is necessary.